(a) PRIOR ART
Hydrocarbon coolants containing fluorine and chlorine such as chlorofluorocarbons and hydrochlorofluorocarbons have conventionally been considered excellent for use as coolants for refrigerators as they are chemically stable and have low toxicity. However, the recent Montreal Protocol decided that the use of chlorofluorocarbons, for example R 12 (dichlorodifluoromethane: Flon 12) shall be totally abolished by the year 1996, because chlorofluorocarbons cause damage to the ozone layer in the stratosphere and thereby contribute to global warming.
While, hydrochlorofluorocarbons such as R 22 (monochlorodifluromethane: Flon 22) have been expected to be used as alternatives to R 12, various countries are conferring in order to abolish the use of R 22 by the early twenty-first century, since there is an uneasiness concerning its damage to the ozone layer.
Based on such circumstances, R 134a and a mixture of R 134a and R 32 have been noted as alternatives to R 12 and R 22 respectively. Also, hydrocarbon coolants which do not contain chlorine in their molecular compositions such as hydrofluorocarbon coolants as represented above have been expected to be used in the feature as coolants.
However, because the polarity of hydrofluorocarbon coolants such as R 134a, R 32 and the like is higher than that of R 12 or R 22, these hydrofluorocarbon coolants have poor compatibility with naphthene mineral oils, alkylbenzenes and the like which have been conventionally employed as lubricants for refrigerators. In order to improve upon said disadvantage as lubricants for use in refrigerators using hydrofluorocarbon coolants, lubricants comprising polyoxyalkylene glycol has been described in U.S. Pat. No. 4,755,316, Japanese Patent Laid-Open No. 03-28296 and the like and lubricants comprising esters have been proposed in Japanese Patent Laid-Open Nos. 03-505602, 03-88892, 03-128991, 03-128992 and the like.
Since small amounts of water exist in compressors of refrigerators, if a compound having an ester linkage is present in the refrigerator oils, there is the problem that the ester linkage may be hydrolyzed to form free acid and said free acid may cause corrosion and sludge.
In order to improve on said disadvantages, the use of a glycidyl ether type epoxy compound and epoxidized vegetable oil as stabilizing agents has been proposed in Japanese Patent Published No. 60-19352, the use of a glycidyl ether type compound having superior compatibility with R 134a has been proposed in Japanese Patent Laid-Open Nos. 03-275799 and 04-55498, and the use of an alicyclic epoxy compound has been proposed in Japanese Patent Laid-Open No. 05-105896.
While polyoxyalkylene glycols are relatively stable against hydrolysis, they have poor heating oxidation stability and lubricating properies. Consequently, when they are subjected to heating oxidation, not only do their molecular weights decrease but they also generate acidic substances which may cause corrosion of materials used in refrigerators. Furthermore, their poor lubricating properties cause some problems such as slight vibrations and an increase in wear of devices in refrigerators.
In order to improve on these problems, for example, Japanese Patent Laid-Open No. 02-102296 discloses refrigerator lubricants which are composed of polyoxyalkylene glycol blended with antioxidant (e.g. phenol-, amine-, phosphorous- and benzotriazole-based one) and a phosphorous-based antiwear agent, and Japanese Patent Laid-Open No. 02-84491 discloses refrigerator lubricants which are composed of polyoxyalkylene glycol monoalkyl ether blended with an epoxy compound and a phosphorous-based antiwear agent.